The present invention relates generally to footwear and is more particularly related to a shoe sole construction wherein the impact energy of the heel strike is absorbed, stored, delayed and then the stored energy is beneficially returned at the right time to aid in the propulsion of the wearer during the propulsive phase of the human gait.
In human locomotion the walking gait cycle is generally considered as comprising two distinct phases: (a) the stance phase, and (b) the swing phase. The beginning of the stance phase is signaled by the strike of the foot against the support surface. At this point of the cycle the foot begins to become loaded with body weight and, in response, pronates, thereby to result in a lowering of the medial longitudinal arch, an outward turning of the foot and an inward rotation of the leg. During this pronation of the foot the bony articulations or joints of the mid and hind foot loosen somewhat in order that the foot can both adjust to the support surface and absorb the mechanical shock of strike and weight bearing. If the strike is at the heel, as compared to the ball or flat-footed, as the plantar surface of the foot rolls forward onto the support surface, at some point subsequent to midstance, the heel begins to invert and the foot begins to resupinate. At this juncture of the stance phase the forefoot is fixed to the support surface, the heads of the first and fifth metatarsals are splayed apart and the foot is in a rigid structural condition and, ideally, in a neutral, that is to say, neither a pronated nor a supinated position. Next, plantar-flexion of the foot begins, the arch becomes rigid and the heel lifts off the support surface, usually with accompanying further supination. The plantar fascia shortens and the toes begin to flex, creating a so-called “windlass effect” whereby the arch is elevated. This constitutes the final or “propulsive” segment of the stance phase immediately preceding the beginning of the swing phase of the gait cycle and the strike of the opposite foot. In the normal swing phase, during which the foot is lifted entirely off the support surface and, therefore, is in a non-weight bearing condition, the ideal foot returns from its supinated position to a neutral position, as do the articulations of the fore, mid and hind foot, all in preparation for the onset of the foot's next stance or weight bearing phase.
Unlike walking, wherein at least a portion of the gait cycle involves double-limb support of the body and a sharing of the body weight therebetween, the running gait cycle includes a third or “float” phase interposed between the stance and swing phases and during which “float” phase both feet are off the ground and following which only one foot receives the entirety of the ground impact forces. The stance or weight bearing phase is substantially shorter than in walking. Thus, in running, the ground contact impact forces imposed upon the anatomy of the foot are substantially greater, usually about three times greater, and require the foot, leg, hip and spinal anatomy to accommodate these stresses over a substantially shorter period of time than in walking. These factors particularly associated with the running gait thus pose an ever present orthopedic threat to the well being of the runner's anatomy of locomotion and have spawned the development of various energy absorptive devices for use in footwear. In general, the known protective devices for runners and athletes take the form of various compressible viscoelastic pads and pillows installed as insole elements under the heel or entire foot of the wearer and which serve to absorb at least a substantial portion of the impact energy of the strike. Usually, these devices act by compression under the loads imposed by the strike and by conversion of this mechanical energy into heat. While effective to various degrees in providing physical protection to the anatomy of locomotion, particularly to that of the foot, the heat generated within these devices can contribute to an uncomfortably warm environment within the wearer's shoe. Moreover, the impact energy absorbed by these devices is simply dissipated and is not returned in any beneficial way to the wearer.
Reference is also made to my earlier U.S. Pat. No. 5,706,589 for a description of one shoe sole construction in which, during the stance phase of the wearer's gait cycle, the impact energy of a heel strike is absorbed, stored and, at least in part, returned to the underside of the forefoot during the propulsive phase of the gait, thereby aiding in the locomotion of the wearer. With this construction, following the propulsive phase of the gait cycle, the sole construction is restored to a condition suitable for absorption and storage of the impact energy of the next heel strike event thereupon. Although this construction represented some improvement in performance, it still did not provide universal application for all styles of running and/or walking.